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If the rows and columns of the CCD are not parallel to the dispersion and slit directions, both the acquisition of the target fields and the subsequent reduction of the data are complicated. Therefore the rotation between the detector and the optical system must be checked, and adjusted if necessary.
The rotation of the CCD with respect to the focal plane can be checked by measuring the X position of the ends of the slit on an image of the focus mask or a long slit with the grism wheel in the clear position. This should be illuminated with a white light source from the calibration system (i.e. a Tungsten lamp) with appropriate neutral density obtained using the ND filters in the comparison system. Note that some DMS routines are confused by pixel values >32000, so ensure that the maximum count is less than this. Typical exposures through a ND5 filter should be roughly 10s in V, and 5s in R. The X position of each end of the slit image can be measured with the DMS routine X-FIND.
The amount of rotation found in a particular exposure depends on BOTH the rotation of the detector with respect to the LDSS-2 optical coordinate frame AND any rotation introduced by incorrect installation of the mask. It has been found that it is possible to reinstall a given mask to an rms accuracy of 0.014 which, for the Tektronix 1024 CCD, is equivalent to 0.24 pixels (0.14 arcsec) over the full extent of the detector. The maximum acceptable rotation error is much larger than this, but it is not difficult to align the CCD such that the slope of the slit image is less than 0.5 pixels from top to bottom. If the slope is greater than this, then the CCD mounting ring should be rotated with respect to LDSS.
The CCD mounting ring is fixed to the mounting ring on LDSS by seven 6mm Allen screws which pass through curved slots in the CCD mounting ring into the spectrograph mounting ring. There are two further Allen screws which hold the LDSS mounting ring to LDSS, these lie on a line through the centre of the mounting ring, and are recessed more deeply than the seven other screws. These two screws should not be loosened. If the seven screws are loosened the cryostat and mounting ring can be rotated by a small amount, the limit is set by the length of the slots in the CCD mounting ring. There is a micrometer gauge fixed to the CCD mounting ring, which can be brought against a block which is fixed to the LDSS mounting ring to give a measure of the relative position angle of the two rings. The cryostat should be rotated by backing off the micrometer; loosening the screws; rotating the cryostat manually; tightening at least three of the screws; then measuring the position angle with the micrometer. The micrometer gauge should not be used to push the cryostat around.
A 1 pixel slope from end to end of the slit will require a movement of the cryostat position angle equivalent to around 0.1mm on the micrometer. The direction of the shift will depend upon which CCD on-chip amplifier is being used, and so will have to be established by trial and error.